WASHINGTON (CNN) -- The 30,000 additional troops deployed to Iraq in January could come home by next July, but further American withdrawals would be "premature," the U.S. commander there told a fractious congressional hearing Monday.

Gen. David Petraeus, U.S. commander in Iraq, reports on progress to Congress on Monday.

"Our experience in Iraq has repeatedly shown that projecting too far into the future is not just difficult, it can be misleading and even hazardous," Gen. David Petraeus said at a joint hearing of the House Armed Services and Foreign Relations committees. "The events of the past six months underscore that point."

The testimony by Petraeus and Ryan Crocker, the U.S. ambassador in Baghdad, was interrupted numerous times by demonstrators.

Rep. Ike Skelton, D-Missouri, the Armed Services Committee's chairman, ordered several protesters arrested for interrupting what he said "may be the most important hearing of the year."

President Bush ordered nearly 30,000 additional troops to Iraq in January as part of a campaign to pacify Baghdad and its surrounding provinces. Petraeus said that campaign has largely met its military goals, reducing sectarian killings by more than 50 percent nationwide and by more than 80 percent in Baghdad.

But Democrats went into the rare joint hearing raising questions about the figures Petraeus would be presenting and putting the focus on the White House, not the general or the ambassador.

House Speaker Nancy Pelosi, D-California, said in a statement, "Today, despite overwhelming evidence that neither goal has been achieved, Gen. Petraeus testified that the surge would last at least until next summer. This is simply unacceptable."

She added, "The president's strategy in Iraq has failed. It is time to change the mission of our troops to one that will promote regional stability and combat terrorism, so that the numbers of our brave men and women in uniform in Iraq can be reduced on a much more aggressive timetable than the one outlined today by Gen. Petraeus."

Senate Majority Leader Harry Reid, D-Nevada, said, "Our enemies around the globe gain great advantage by having the United States mired in an Iraqi civil war."

Keeping troops in Iraq until at least July 2008 is "not in the national interest of the United States," he said.

"The longer we keep over 130,000 troops in Iraq, the less incentive Iraqis have to engage in the needed political reconciliation and the longer we avoid dealing with several pressing threats to our national security," namely that Osama bin Laden remains at large, Reid said. See how much the war is costing »

Skelton, D-Missouri, warned Petraeus and Crocker they were staring into a deep well of skepticism and frustration on Capitol Hill.

"I hope, Gen. Petraeus and Ambassador Crocker, that you can persuade us that there is substantial reason to believe that Iraq will turn around in the very near future," Skelton said. "But you have the burden of answering these fundamental questions to those of us who have been watching Iraq for years. And every promising development has so far not turned out to be the solution we hoped."

Skelton said while U.S. troops have made tactical gains, particularly in the Sunni Arab province of Anbar, Iraqi leaders "have not stepped up to the challenge" of reaching a political settlement to the four-year-old war.

But Crocker said political gains in Iraq have been more pronounced at the provincial level than nationwide, and Iraqis have been taking practical action toward reconciliation even though they have been unable to set those steps into law.

Militarily, U.S. analysts have said the armed services will be unable to maintain the current level of more than 160,000 troops beyond next spring.

Petraeus said one Marine expeditionary force would be coming home in September, and an Army brigade is slated to return in mid-December. He said four more brigades would be withdrawn by July 2008, leaving about 130,000 U.S. troops in Iraq.

Details from Hearing

But Petraeus said decisions on further withdrawals should be put off until mid-March.

"We will, no later than that time, consider factors similar to those on which I based the current recommendations, having by then, of course, a better feel for the security situation, the improvements in the capabilities of our Iraqi counterparts, and the enemy situation."

"The fact remains that the administration has sent you here today to convince the members of these committees and the Congress that victory is at hand," said Rep. Tom Lantos, D-California, the chairman of the Foreign Affairs Committee. "With all respect to you, I don't buy it."

The Democratic leadership in Congress has been unable to force further reductions. In May, President Bush vetoed a call for U.S. combat troops to leave Iraq by April 2008.

Petraeus commanded the Army's 101st Airborne Division during the 2003 invasion and spent a year training a new Iraqi military in 2004. He is considered a leading military scholar on counterinsurgency, and has tried to put those tactics to work to pacify Baghdad and its surrounding provinces.

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He said military progress has been uneven, but the "surge" campaign launched in January is meeting its goals. And he said U.S. objectives in Iraq can still be met, "although doing so will be neither quick nor easy."

Petraeus said he wrote his testimony himself, and it had not been cleared by or shared with anyone in the White House or Pentagon. He said U.S. intelligence agencies consider the military's numbers "the most accurate and authoritative in Iraq."

"We endeavor to ensure our analysis of that data is conducted with rigor and consistency, as our ability to achieve a nuanced understanding of the security environment is dependent on collecting and analyzing data in a consistent way over time," he said. See photos of the day's events »

But David Walker, head of the nonpartisan Government Accountability Office, told the Senate Armed Services Committee on Friday that he is not comfortable with the methodology used to track the death toll driven by sectarian violence.

Rep. Duncan Hunter of California, the ranking Republican on the Armed Services Committee, criticized Democrats for questioning the general's credibility.

"We've asked you for an independent assessment, and frankly, Mr. Chairman, the idea that we have spent the last week prepping the battlefield by attacking the credibility of the messenger is something that I think goes against the tradition of this great House."

Republicans also blasted a full-page advertisement in the New York Times, placed by the anti-war activist group MoveOn.org, that questioned whether Petraeus was "cooking the books for the White House."

Rep. Ileana Ros-Lehtinen of Florida, the ranking Republican on the Foreign Affairs Committee, called on Democrats to publicly denounce the ad and apologize.

Hunter said Petraeus "is going to testify with an independent candid view, the one thing we expect from all of our military officers."

That drew a frosty response from Skelton, who said, "The capability, the integrity, the intelligence and the wisdom of our two witnesses requires nothing but admiration from me and from others who are about to receive their testimony."E-mail to a friend